The present invention relates to an improved mechanical shaft joint boot made of a thermoplastic elastomeric resin, this boot having a bellows between a large-diameter ring and a small-diameter ring and, more particularly, to a mechanical shaft joint boot (hereinunder referred to as "boot") suitable for a shaft joint of a constant velocity type for automobiles. In this specification, "the thickness of a crest" means the thickness of the top of a crest, i.e. its optical thickness.
Such type of a boot is compressed a little toward a shaft when it is mounted on a joint. Therefore, it is generally considered that uniform dispersion of the deformation of the boot in the bellows will improve the durability of the boot and, as shown in FIG. 1, for example, the crests of the bellows 7 are formed in such a manner that the first cresh (hereinunder the crests are numbered first, second, . . . in order, from the small ring of the bellows) having the smaller diameter has a larger thickness than the other crests. Therefore, when the boot shown in FIG. 1 is mounted on a joint, the pitch of each crest is made shorter by substantially (L-l)/5, as is indicated by the broken line in FIG. 1, wherein L represents the length of the boot before it is mounted on the joint, and l the length of the boot after it is mounted on the joint.
The reference numeral 3 denotes a large-diameter ring.
As a result of investigation on the durability of such a boot, the present inventors found the following problems:
When such a boot is mounted on a joint, since a pitch between each crest is made shorter, interference (contact) between the crests and a bending (kinking) phenomenon are likely to be produced at the crest on the side of the large-diameter ring 3 during the cross movement (i.e. arcuate flexing) of the joint. As a result, in particular, the boot shown in FIG. 1 involves a risk of producing a damage due to wear and lowering the durability.